The invention relates to a circuit arrangement with a filter quadripole, (four terminal device) Such circuit arrangements are used, for example, in receiver circuits as used, for example, for pagers.
Receiver circuits are generally to satisfy certain requirements as regards the turn-on time. The term turn time is then used for the period of time between applying the supply voltage to the receiver circuit and the reaching of an operating state in which a perfect processing of the signals received by the receiver circuit is possible. In battery-operated receiver circuits, the turn-on time has a considerable effect on the useful life of the battery.
For example, pagers need to have turn-on times in the range of milliseconds. It must be born in mind that this turn-on time for the whole pager is formed by the sum of the turn-on times of individual circuit components and sub-sets, so that the duration of the turn-on time for receiver circuits in pagers is required to be restricted to a few microseconds.
Alternatively, receiver circuits having a low intermediate frequency, more particularly, in the kilohertz range, require high-pass filters for d.c. decoupling in a high-gain signal path for the received signal that is to be processed which high-pass filters have a cut off frequency that is selected to be low with an accordingly low intermediate frequency i.e. have a high time constant. During the turn-on time i.e. while the receiver circuit is taken into operation, the signal regularly shows voltage jumps produced by this receiver circuit, which jumps may be the result of asymmetries, self-reception but also other influences. In the case of self-reception, the signal produced by the local oscillator of the receiver circuit overcouples on its antenna input and, in consequence, is mixed with itself to an offset d.c. voltage in the mixer connected to the local oscillator. In the build-up time of the oscillator, during the turn-or time when the receiver circuit is taken into operation, this offset d.c. voltage appears as a voltage jump on the output of the receiver circuit. This voltage jump is amplified with high gain provided for the signal to be received in the receiver circuit and, as a result, overloads the high-pass filters which the signal passes trough in the signal path. Depending on the amplitude of this voltage jump (noise voltage) and the time constant of the high-pass filters, the decay time of the noise voltage on the output of the receiver circuit (or of the high-pass filters, respectively) may be several milliseconds. For the requirements to be made on the pagers and their receiver circuits, the decay time is much too long.